This invention relates to the photoscreening of slot-mask, stripe-screen type color cathode ray tubes and in particular concerns method and apparatus for improving the width uniformity and edge definition of the stripes of phosphor material and, in certain tubes, the stripes of black material interposed between the stripes of phosphor material.
Conventional color cathode ray tubes of the slot-mask, stripe-screen type include a curved, approximately rectangular faceplate which has a concave inner surface and which supports in spaced relationship thereto a shadow mask having a correspondingly curved central portion in which is formed an array of spaced columns of slots oriented parallel to the minor axis of the faceplate.
Certain commercial tubes of the slot-mask, stripe-screen type have so-called "positive tolerance" between the beam landing areas and the phosphor stripes -- that is, a condition in which the beam landing areas (defined by the mask slots) are narrower in the horizontal (scanning) direction than the phosphor stripes by an amount sufficient to provide a tolerance against misregister between the beam landing areas and the phosphor stripes. A second class of tubes of the slot-mask, stripe-screen type have a "negative tolerance" condition in which the beam landing areas are wider in the horizontal direction than the phosphor stripes by a predetermined amount sufficient to provide a misregister tolerance.
In the tubes of the negative tolerance type it is a universal practice to interfill the spaces between the narrow phosphor stripes with a black material in order to improve the contrast of the tube and make possible a marked improvement in brightness. It is more difficult to form the stripes in tubes of the negative tolerance type than in tubes of the positive tolerance type, since the phosphor stripes must be formed narrower than the slots in the mask (the mask is conventionally used as a photographic stencil during the photoscreening process).
Either of two methods is commonly employed to produce screen stripes which are narrower than the mask slots. In one process, the so-called "etchback" or "post-etch" process, a shadow mask having slots smaller than their ultimate size is used during the photoscreening process to form the desired narrow phosphor stripes. Subsequently, the mask slots are enlarged to a size appropriate to form beam landing areas larger than the phosphor stripes.
it is now more common practice to form the narrowed phosphor stripes by a strictly photographic process wherein a light source is used which has such dimensions as to form light penumbras, which penumbras can be effectively utilized to create exposure patterns which are narrower than the mask slots.
It is common practice to use the same penumbra photoscreening techniques to form continuous, full-height phosphor and matrix stripes (stripes which are unbroken from top to bottom of the screen, including behind the "tie-bars" between the mask slots. Continuous phosphor stripes are desirable to avoid any problem of vertical misregister of electron beam landing area and phosphor element. It is not an uncommon practice in the industry, however, to form phosphor stripes which are discontinuous, with the areas behind each tie-bar being covered with black material rather than phosphor. The teachings of the present invention may be applied to tubes having either continuous or broken phosphor stripes. As used herein, the term "stripe" is intended to be generic to both species of stripes.
It is standard practice in the photoscreening of slot-mask, stripe-screen tubes to employ a line source of UV (ultraviolet) radiation, the axis of which source is aligned parallel to the minor axis (commonly termed the "Y" axis) of the faceplate to be screened. It has been found, however, that due to the nonparallelism of the line source with the slots in the mask in all regions of the faceplate except the center and along the major and minor axes, there results an unwanted rotational displacement of the individual slot images projected on the faceplate inner surface. This results in a nonaligned overlapping of the projected slot images, with a consequent uniformity in the width of the stripes and a poor definition of the stripe edges. This problem is especially severe in the corners of the faceplate. Ragged, nonuniform screen stripes produce undesirable degradation of color purity and a poor screen appearance.
A number of prior art approaches have been developed to overcome the afore-described stripe formation problems. The most commonly employed approach to solving these problems, expounded by German Pat. No. 2,408,993 and U.S. Pat. Nos. 3,888,673 and 3,890,151, involves moving the light source, the light dispersion correction lens(es), and/or an auxiliary scanning mask, during the photoexposure operation. The prior art approaches utilizing a moving light source, lens, and/or scanning mask are apt to suffer, however, from undesirable complexity, inflexibility, and unreliability. More importantly, approaches which utilize a scanning mask to block parts of the screen while other parts are being exposed, are extremely wasteful of exposure light, necessitating very long exposure times and resulting in increased screening costs.
As will be explained in great detail hereinafter, by this invention, in its apparatus context, there is provided screening exposure lighthouse apparatus for assisting in the photoscreening of slot-mask, stripe-screen type color CRT's which utilizes a simple negative meniscus lens, or one equivalent in effect, to rectify to a major extent the afore-described stripe width nonuniformity and edge definition problems.
The prior art U.S. Pat. No. 3,587,417 discloses a light-house for photoscreening color cathode ray tubes which includes an anamorphic lens to bring into coincidence the apparent position of the light source virtual image for all locations on the faceplate. More particularly, the U.S. Pat. No. 3,587,417 shows a lens which has a negative meniscus characteristic in a plate parallel to the major axis (the "X" axis) of the faceplate but which has no dioptric power parallel to the minor ("Y") axis of the faceplate. The apparatus disclosed in the U.S. pat. No. 3,587,417 patent is designed for use with dotscreen type tubes having shadow masks with circular holes and would have no applicability to the solution of the afore-described stripe degradation problems associated with the photoscreening of slot-mask, stripe-screen tubes using line sources.
A German Pat. No. 1,958,521 suggests a negative meniscus lens for use in redistributing the light intensity from a point source across a faceplate assembly of the type having a dot screen and circular-apertured shadow mask. Experiments with a mensicus lens of the type described in the 1,958,521 patent, used for the purpose intended, were found to be unsuccessful. The 1,958,521 patent is devoid of any disclosure of the use of a lens in the photoscreening of slot-mask, stripe-screen tubes for the purpose of rectifying the afore-described ragged stripe problem.